• Hyperthyroidism developed in 70 patients with diabetes mellitus. Nine patients had masked hyperthyroidism with weight loss as the main sign. Diabetic control deteriorated in 28 of 48 insulin-dependent patients and in four of 22 taking hypoglycemic agents orally. Recurrent ketoacidosis occurred in two. Insulin requirements increased in 11 of 48 by 25% to 100% (mean, 50%). Following treatment of the hyperthyroidism, insulin requirements decreased in 13 of 48 by 20% to 100% (mean, 35%). Four of 22 patients were changed from orally ingested hypoglycemic drugs to insulin. Because of similar symptoms, underlying hyperthyroidism must be considered in any patient whose diabetes is poorly controlled.